1. Field of the Invention
The invention relates to a means for supporting a mask holding frame for suction and attachment of a mask which is used for an exposure device, a proximity exposure device and the like. The invention especially relates to a means for supporting a mask holding frame with a device which adjusts the vertical angle of the mask holding frame and holds the mask surface perpendicular to the optical axis.
2. Description of Related Art
FIGS. 9 and 10 each schematically show a conventional means for supporting a mask holding frame. FIG. 10 is a cross-sectional view taken along line A--A in FIG. 9. In the drawing, a mask holding frame 1 and a base 2 are shown which, together form a mask carrier.
Mask holding frame 1 has a means which suctions a mask (not shown) by vacuum or similar methods. Furthermore, in the three corner areas A, B, and C of mask holding frame 1, there are screw openings in which ball plungers 3 are installed. In the vicinity of the openings for the ball plungers 3, there are through openings which are penetrated by mounting pins 4. On the respective mounting pin 4, are a washer 6 and a compression spring 5 (disk spring or helical spring).
In base 2 are V-sleeves 7 which are engaged by ball plungers 3, and screw openings 8 to which mounting pins 4 are joined. Mask holding frame 1, as shown in FIG. 10, is installed on base 2 by means of the mounting pins 4 and ball plungers 3, and is prestressed by compression springs 5 toward the base 2.
By the arrangement of ball plungers 3 in three corner areas A, B, and C the height of ball plungers 3 which are located on two orthogonally intersecting axes can be adjusted. Thus the vertical angle of mask holding frame 1 can be adjusted. Therefore the adjustment process can be simplified.
In FIGS. 9 and 10, the position adjustment of mask holding frame 1 is done in the manner described below.
(1) In mask holding frame 1, a mask (not shown) is attached by means of a vacuum chuck. PA1 (2) By screwing ball plungers 3 up or down the vertical angle of mask holding frame 1 with respect to base 2 is changed and adjusted such that the mask surface becomes perpendicular to the optical axis of the exposure light of the exposure device. PA1 (3) Mounting pins 4 are screwed down and mask holding frame 1 is attached. PA1 However, if the force with which mask holding frame 1 is pressed in the direction toward base 2 is increased, the force with which ball plungers 3 are loaded also becomes greater. In the arrangement shown in FIGS. 9 and 10, the points which are exposed to the force of the mounting pins 4 differ from the points which are exposed to the force of ball plungers 3. If the above described pressing force becomes greater, distortions occur in mask holding frame 1 between the points at which mounting pins 4 are installed and the points on which plungers 3 are installed. Therefore, the defect of bending of the mask holding frame cannot be eliminated solely by increasing the pressing force.
This holding means has a simple configuration, and can be easily produced and adjusted. Furthermore, mask holding frame 1 is always pressed with a constant force in the direction toward base 2 by the compression springs 5 installed on mounting pins 4. The distance between mask holding frame 1 and the base 2 does not change. This holding means therefore has the feature that the tilt of the mask relative to the optical axis does not change even after use over a long time.
Recently, there has been a need for holding a large mask (for example, one with dimensions of 400 mm.times.400 mm) which is used for exposure of a liquid crystal substrate. The mask holding frames are therefore becoming larger and larger. As a result, the arrangement shown in FIGS. 9 and 10 has the following defects:
(1) When the mask holding frame 1 becomes larger, in the arrangement shown in FIGS. 9 and 10, the corner area D which is provided neither with ball plunger 3 nor V sleeve 7, is bent or tilted by its own weight, as shown in FIGS. 11(a) or 11(b).
When bending/tilting occurs in mask holding frame 1, the mask installed therein is also bent or tilted. The distance between the mask and the workpiece is therefore no longer constant. The mask surface is no longer perpendicular to the optical axis. Furthermore, in the mask image which is projected onto the workpiece there are points at which no imaging takes place. Therefore, the exposure accuracy is reduced.
(2) To eliminate the tilt of the holding frame for a large mask, in the arrangement shown in FIGS. 9 and 10, the spring constant of the compression springs 5 of mounting pins 4 or the number of disk springs can be increased, and thus, the force is increased with which the three corner areas A, B, and C of mask holding frame 1 are pressed in the direction toward base 2, so that corner area D which is not provided with a ball plunger is raised without being tilted.
(3) In a proximity exposure device, for parallel adjustment of the mask and workpiece relative to one another with a constant distance, a distance setting process is used which, for example, is described in Japanese patent disclosure document HEI 7-74096. In the distance setting process, the workpiece is moved upward, pushed against the bottom of the mask and then moved farther upward. The overall surface of the workpiece is brought into contact with the mask, and the mask and the workpiece are positioned parallel to one another (hereinafter this actuation is called "parallel arrangement"). While maintaining this state, the workpiece is separated from the mask only to a given degree and the two are set parallel to one another, and at the same time, with a constant distance relative to one another.
In the case of distance setting by this process, in the arrangement shown in FIGS. 9 and 10 mounting pins 4 for attaching mask holding frame 1 are located only in three corner areas A, B, and C. When the workpiece is pressed from underneath and brought into contact with the mask to bring the entire surface of the mask and the entire surface of the workpiece into contact with one another, corner area D of mask holding frame 1 is bent upward. Therefore, parallel arrangement of the mask and workpiece relative to one another cannot be done correctly.